This invention relates to improving the heat distortion temperature (HDT and tensile strength of glass fiber reinforced (GFR) poly(vinyl chloride) (PVC) without vitiating other physical properties of the GFR PVC. More specifically, this invention relates to the improvement of the foregoing properties, inter alia, by blending the PVC with a particular copolymer, reinforcing it with specifically sized glass fiber, and incorporating these components with a unique mixing procedure.
It is known that, with an aminosilane coupling agent and the correct choice of sizing agent, glass fibers may be so strongly bonded to the PVC that a GFR PVC composite formed therewith fails in cohesive failure. By "cohesive failure" we refer to failure of a sample of GFR VC resin due to tearing of resin from resin, rather than tearing of resin from the glass surface ("adhesive failure"). Thus, cohesive failure is predicated upon the resin's properties rather than upon the bond between resin and glass.
Details of the mechanism of the reaction thought to be responsible for the improved physical properties of the aforementioned GFR PVC composite are taught in U.S. Pat. No. 4,536,360 to Rahrig, D., the disclosure of which is incorporated by reference thereto as if fully set forth herein. However, the utilization of the Rahrig GFR PVC in applications requiring operation of an article at a relatively high temperature under load, is restricted by the limitation of the relatively low HDT of the GFR PVC. Though the HDT of PVC is improved by the presence of the glass, this improvement of HDT in the range from about 10% by weight (wt) to about 30% by wt of glass, based on the total wt of the GFR PVC, is only marginal. For example, the HDT of GFR commercial grade Geon.sup.R 86 PVC, reinforced with 10% glass is about 165.degree. F. (74.degree. C.), and with 30% glass is about 170.degree. F. (76.7.degree. C.).
An improvement was subsequently made relating to a wider choice of film formers to catalyze the thermal dehydrohalogenation of the VC homopolymer at the fiber-resin interface so as to generate allylic Cl moieties in chains of the homopolymer, which moieties react with the amine groups of the aminosilane. Details of this improvement are taught in copending U.S. pat. application Ser. No. 897,437 filed Aug. 18, 1986, the disclosure of which is incorporated by reference thereto as if fully set forth herein. However, a wide spectrum of film formers fell far short of providing a noticeable improvement in HDT, or resulted in a noticeable loss in tensile strength, degradation of spiral flow, etc. and failed to fill the need for an inexpensive, durable and rugged GFR PVC composite with relatively higher HDT, at least equivalent tensile strength, excellent wet strength and only a small loss in impact strength, compared to the aforementioned prior art composites. By "equivalent tensile strength" we mean that the measured tensile strength is not less than 90% of that of a similar prior art composite.
By relatively higher HDT we refer to a HDT of at least 80.degree. C. (176.degree. F.) which is about 3.degree. C. (5.4.degree. F.) higher than the HDT of any commercially available GFR PVC produced according to the '360 patent. The HDT of general purpose grade injection molding PVC resin is about 74.degree. C. (165.degree. F.), and by reinforcing it with 30% glass fibers sized as in the '360 patent, the HDT is about 76.7.degree. C. (170.degree. F.). It must be borne in mind that such GFR PVC having a HDT of less than 170.degree. F. has inadequate tensile strength and creep resistance under load for general purpose applications where the GFR article is exposed to harsh environmental conditions which may reach to about 180.degree. F. Such a temperature is reached in a closed automobile left in the sun on a summer day in the southern U.S., in northern Africa, or in southeast Asia, temperatures in the range from 76.7-79.4.degree. C. (170-175.degree. F.) being more common than those in the range from 79.4-83.2.degree. C. (175-180.degree. F.). Thus, every degree of improvement in HDT resulted in being able to assure the usefulness of a shaped article made from the GFR PVC blend at a higher temperature than prior art materials, without any substantial loss in desirable physical properties. In particular, it was essential that the tensile strength of the Rahrig composites at least be matched, if not improved. Since there is a need for GFR PVC articles which will withstand each progressive incremental degree above 170.degree. F. for such harsh environmental exposure, the subject matter of this invention derived from a deliberate and concerted effort to fill that need.
Since the improvement sought primarily related to maintaining the chemistry at the surface of the glass fiber, which chemistry was known to be effective, it was logical to search for a copolymer which was miscible with PVC. Since the improvement also sought to improve HDT it was logical to seek a copolymer which contributed high HDT to a blend in which it was a component. But the presence of the copolymer could not adversely affect the bonding of the PVC to the glass fibers.
From the foregoing related disclosures it was known that effective bonding relied upon there being a sufficient number of runs of 10 or more C atoms in VC chains to generate an allylic chlorine (Cl) moiety in the VC chain, represented thus: EQU --CH.sub.2 --CHCl--CH.dbd.CH--CH.sub.2 --CHCl--CH.sub.2 --CHCl--
under thermoforming conditions. But it was not known what effect the presence of a blended copolymer would have in this regard, though it was evident that its structure and the relative number of copolymer chains present, would be determinative.
Therefore the choice of the copolymer to be blended with the PVC required that the copolymer not interfere with the ability of the PVC to generate the necessary allylic Cl moiety.
From the foregoing related disclosures it was also known that the enhanced properties of the improved composite (relative to GFR PVC composites which failed in adhesive failure) required the use of an aminosilane coupling (or keying) agent (sometimes referred to as `finish`) which is essential, in combination with certain polymeric film formers used in the production of glass fibers, most preferably from E glass.
Therefore the choice of the copolymer to be blended with the PVC required that the copolymer not interfere with the ability of the aminosilane to provide the necessary chemistry to perform its designated task.
From the foregoing related disclosures it was also known that the enhanced properties of the improved composite (relative to GFR PVC composites which failed in adhesive failure) required the use of a particular "size", namely one which has sufficient basicity as evidenced by a Cl(2p)/C(ls) peak ratio of at least 0.91.
Therefore the choice of the copolymer to be blended with the PVC required that the copolymer not interfere with the basicity of the film former used to provide the necessary chemistry to perform its designated task.
Since the chemistry occurring at the surface of the glass fiber was critical to the successful reinforcement of any blend, the question which presented itself was not whether, but how, that chemistry would be affected by the presence of any copolymer known to provide high HDT at the elevated processing temperature necessary to thermoform, and specifically extrude, or injection mold, PVC resin composites, not to mention that any effect on this chemistry would further be complicated by the presence of a stabilizer without which a PVC resin cannot be effectively thermoformed.
Finally, assuming the "correct" polymer was found for producing the desired GFR PVC blend with improved HDT, one had to recognize that there may be a decrease in tensile strength, spiral flow and impact strength, rather than an overall improvement in any one property, particularly tensile strength.
In view of the foregoing, it seemed logical to search for a suitable copolymer first among those known to be miscible with PVC, and it was convenient to search among these copolymers for those which could be prepared from readily available monomers, and those which were commercially available. In this framework it was not long before the commercially available PVC blends with copolymers disclosed in U.S. Pat. No. 3,053,800 to Grabowski et al claimed our attention. What received even more of our attention was that, despite the clear teaching that the excellent HDT and impact strength of their blends were most useful for rigid shaped articles for use under conditions where high environmental stability was required, there was a singular lack of any suggestion that the blend may be reinforced with any reinforcement of any kind, in any way. This lack of what should have been an opportunity to explore technology known at the time to provide reinforcement of polymers generally, to make an improvement directly in line with the purpose for which the blend was found most useful, led us to believe that the reinforcement of such blends was seriously circumscribed.
Further inspection of the '800 patent for assistance indicates that in the three-component blend of (i) PVC with (ii) the copolymer of alpha-methyl styrene ("AMS"), styrene ("S") and acrylonitrile ("AN") (the copolymer is referred to as alpha-SAN, for brevity), and (iii) the graft copolymer of the polybutadiene latex (rubbery phase), the alpha-SAN and rubbery phases are each critical for the formation of the blend with PVC. Since the rubbery phase of graft copolymer (of AN and S grafted to a PBD latex) is known to exist as a discontinuous rubbery phase in PVC, it was concluded that in a blend of the three components, alpha-SAN provided the continuous phase while PVC and the graft copolymer existed as the discontinuous phases. It seemed highly improbable that a relatively small amount of alpha-SAN copolymer by itself (that is, without the graft copolymer) blended with a major amount of PVC might provide an essentially single phase having a significantly improved HDT, that is, at least 80.degree. C. (176.degree. F.).
It was in the foregoing framework that we discovered that a GFR blend of PVC and alpha-SAN copolymer would provide a substantially single phase which could be reinforced with a particularly sized glass fiber to provide a GFR composite of the blend having a HDT of at least 80.degree. C. (176.degree. F.). In such a composite, the PVC preferentially wets the glass to produce desirable cohesive bonding resulting in improved tensile strength, only if the amount of alpha-SAN copolymer in the blend was maintained in the narrowly defined range of from 15 to about 40 phr (parts by weight (wt) per 100 parts of blended resin). An amount less than 15 phr produces no substantial improvement of HDT, and an amount greater than 40 parts produces a brittle composite with unacceptably low impact strength.
Though impact modification of PVC by using a specific combination of impact modifiers, namely the ABS graft copolymer and the alpha-SAN copolymer, was the sole thrust of the '800 patent, our invention provides a PVC blend with excellent HDT and tensile, in addition to good spiral flow and retention of tensile and impact strength after long exposure to water. These properties, namely tensile and resistance to water, derive from the cohesive bonding we obtain, which only Rahrig suggested.
Having thus arrived at a composition of a GFR PVC blend which would meet the criteria for use under harsh environmental conditions when injection molded or compression molded without an impact modifier, it became evident that an extrusion grade blend would require a compatible impact modifier. Since it was known that the acrylonitrile, butadiene, styrene graft copolymer of the '800 patent provided the impact performance in that blend because there was good "wetting" of the non-rubbery phase it seemed that it would lend itself particularly well as a suitable impact modifier. However, we found that the wetting was not good enough to provide the desired morphology and chemical reactions required to produce reliable and reproducible cohesive failure in a composite. Hence it became necessary to provide a more suitable impact modifier, which we have done.